With the recent depletion of resources such as petroleum and natural gas, oil wells and gas wells have been developed at great depths at which development has not been conducted and in a sour environment containing hydrogen sulfide (H2S) and having a high corrosive effect. Therefore, oil country tubular goods for drilling and linepipes for transport have been required to have excellent resistance to SSC in a sour environment containing hydrogen sulfide (H2S) and a high yield strength of 110 ksi grade or higher.
The resistance to SSC is typically evaluated in conformity with, for example, Non Patent Literature 1, that is, a method (Method A) specified in NACE TM 0177. This method is a test method in which a particular stress is applied to a round bar tensile test specimen (refer to FIG. 2) sampled from a steel by machining in a standard solution (e.g., H2S-saturated 5% NaCl (salt)+0.5% CH3COOH (acetic acid)) and whether failure occurs before 720 hours pass is evaluated. The round bar tensile test specimen used includes a parallel section, a shoulder section, and a grip section as illustrated in FIG. 2.
The evaluation target is a parallel section. In the test, for example, a stress of 80 to 95% of the specified minimum yield strength (SMYS) of the steel is applied to the parallel section. The parallel section has the smallest diameter in the round bar tensile test specimen, and the stress applied is higher than those in other sections. Therefore, when SSC failure occurs, the failure occurs near the center of the parallel section so that proper evaluation can be performed in the parallel section.